The present invention relates to an ytterbium sputtering target and a method of producing such a target capable of reducing the irregularities (gouges) existing on the target surface after the final finish processing of such target material.
Ytterbium (Yb) is a rare-earth element, and is contained in the earth's crust as a mixed composite oxide mineral resource. Rare-earth elements are called “rare-earth” elements because they are separated from relatively rare minerals, but they are not that rare in light of the overall earth's crust.
Ytterbium is a gray metal having an atomic number of 70 and an atomic weight of 173.0, and comprises a cubic close-packed structure at normal temperature. Ytterbium has a melting point of 819° C., boiling point of 1194° C., density of 6.97 g/cm3, its surface is oxidized in the atmosphere, and is also soluble in acid. With rare-earth elements, compounds having an oxidation number of 3 are generally stable, and ytterbium is also trivalent, but certain rare-earth elements are bivalent. The present invention covers all of the above.
In recent years, research and development for using ytterbium as an electronic material such as a metal gate material and High-k material are being promoted, and ytterbium is a metal that is attracting attention.
Under normal circumstances, the foregoing electronic material is formed by way of sputtering. Sputtering is a method of applying DC voltage or high-frequency voltage between a substrate and a sputtering target while introducing inert gas (primarily Ar gas) into the vacuum to cause the ionized Ar to collide with the sputtering target, and deposit the discharged target substance onto the substrate.
As a method of producing an ytterbium sputtering target, proposed is a method of cutting a high purity ytterbium ingot into a prescribed size, and grinding and polishing to produce a high purity ytterbium target (for instance, refer to Patent Document 1).
Nevertheless, owing to ytterbium's fragile and soft feature at room temperature, narrow irregularities (gouges) are formed in the vicinity of either side of the surface cut marks when machined chips (mostly thin, linear chips) are separated in bite cutting surface. These irregularities (gouges) are visually recognizable in size, to be more precise, approximately tens of μm to hundreds of μm
However, these fine irregularities (gouges) actually cause serious problems. When performing bite cutting, the irregularities (gouges) are crushed pursuant to the bite feed, and leave numerous flaws on the ytterbium target surface. In particular, when the target material is of a high purity, ytterbium at room temperature is so soft that it can be scratched with a human nail, and this will become a serious problem.
Moreover, when forming the target in a disk shape, there is a problem in that fine scratches are left in a concentric pattern on the target surface since the target surface is cut concentrically.
As conventional technology, disclosed is technology of finishing the surface of a W alloy sputtering target to a gloss surface of an extremely small size with minimal irregularities by a machined finish such as grinding to eliminate the work-defect layer containing minute cracks and the like that occur during the machining (for instance, refer to Patent Document 2).
Also disclosed is technology of considerably reducing the particles that are generated during sputtering in a silicide sputtering target by partially eliminating the work-affected layer of the surface and making the surface roughness to be 0.05 to 1.0 μm (for instance, refer to Patent Document 3).
Moreover, disclosed is technology of reducing the generation of particles during sputtering by performing finish processing on the sputtered face by subjecting the sputtering target surface to precision cutting, and selecting the processing conditions to achieve a centerline average roughness of Ra=0.2 μm and a work-affected layer thickness of 15 μm (for instance, refer to Patent Document 4). However, the object of each of the foregoing technologies is to obtain a target having a surface with minimal surface roughness and low processing strain by devising the processing conditions in the machining of the sputtering target surface.
Meanwhile, as an example that focused on controlling the properties of the target surface, disclosed is an invention of making Vickers hardness to be 30 to 80 and keeping the variation thereof to be within 20% in order to prevent the generation of abnormal discharge in a sputtering target having Al as its primary component (for instance, refer to Patent Document 5). Nevertheless, the object of the foregoing invention is to overcome the problems that arise during the sputtering deposition using such a target by controlling the properties after the target surface has been processed, and Patent Document 5 does not offer any object or motive for resolving the “gouges” that arise at the manufacturing stage (surface machining), which is a problem that is unique to a totally different material of an ytterbium target.
The present inventors devised the processing conditions as with the conventional examples in order to inhibit the generation of the foregoing irregularities (gouges); that is, by attempting to adjust the cutting edge angle, cutting edge depth and cutting speed of the bite during the final finish processing. However, even though the irregularities of the gouges came to be smaller by reducing the cutting edge angle and narrowing the cutting edge depth, the result was insufficient.
Specifically, the number of gouges will decrease when the cutting speed is lowered, but the irregularities tend to be larger. Meanwhile, when the cutting speed is increased, there is a problem in that the number of gouges will increase and that the machined chips will ignite. Thus, it was not possible to reliably inhibit the generation of gouges simply by adjusting the bite cutting conditions.
These irregularities (gouges) cause the generation of particles during sputtering. Even assuming that the gouges are ground with a scraper or the like after the machining process and rolling is performing at a pressure of roughly 0.2 mm in order to make the irregularities unnoticeable, this is insufficient in inhibiting the generation of particles and, contrarily, there were cases where the generation of particles increased.
In addition, since ytterbium is a chemically active metal, it easily reacts with the oxygen, moisture and carbon dioxide in the atmosphere. Thus, for example, if finish processing of wet polishing is performed on the target surface, there is a problem in that an oxide film or a carbonized film is formed, contamination from oxygen or carbon or from the abrasive will arise, and plasma cannot be generated when starting the sputtering process.    [Patent Document 1] Japanese Patent Application No. 2007-274808    [Patent Document 2] Japanese Patent Laid-Open Publication No. H3-257158    [Patent Document 3] Japanese Patent Laid-Open Publication No. H6-322529    [Patent Document 4] Japanese Patent Laid-Open Publication No. H11-001766    [Patent Document 5] Japanese Patent Laid-Open Publication No. 2004-204284